In projection display systems the function of the illumination system is to provide uniform illumination to a spatial light modulator (SLM). Typically the illumination system in a projection display consists of a light source such an arc lamp or light emitting diodes (LEDs), a homogenizer of some type for providing homogenized light at the SLM, and a relay lens for providing a beam having a desired size, shape and telecenetricity.
In some projection display systems, liquid crystal spatial light modulators are used, and it is desirable to illuminate such modulators with polarized light. A polarizer can be added to the illumination system to filter the light, so as to provide a polarized beam to the SLM, however this method has an associated loss of 50% of the light, as one polarization state is absorbed or otherwise lost. Illumination systems utilizing polarization recovery techniques seek to take this lost light and covert it to the desired polarization state so it can be recovered and utilized, thus increasing the efficiency of the system and hence the brightness of projection. Prior art methods of performing this polarization recovery and conversion include the use of lenslet arrays and a polarization conversion array (PCA) or the use of a polarization converting light pipe (PCLP).
Projection displays incorporating transmissive liquid crystal SLMs typically use a lenslet array and a PCA; some reflective liquid crystals SLMs also known as liquid crystal on silicon (LCoS) also use a lenslet array and PCA. Prior art FIG. 1 of the accompanying drawings is a schematic view of the essential portions of another liquid crystal projector disclosed in U.S. Pat. No. 6,139,157. In FIG. 1, light emitted by a lamp 201a is reflected toward an image display element 207 by a reflector 203, and enters a first lens array 201b comprising a plurality of lenses arranged into the form of a grating. The aforementioned light is condensed near the lenses of a second lens array 202 similar in construction to the first lens array 201 and comprising lenses having the same degree of focal length as the interval between the first lens array 201 and the second lens array 202 and arranged into the form of a grating by the lenses of the first lens array 201, and is caused to be transmitted through the lenses of the second lens array 202, whereafter it enters a polarization converting element 204.
The light beam which has entered the polarization converting element 204 is separated into different polarized components (S component and P component) by a polarization separating surface 204a, and S-polarized light reflected by the polarization separating surface 204a is reflected by a reflecting mirror 204b and is transmitted through a half wavelength plate 205, whereby it is converted into the same polarized state as the P-polarized light transmitted through the polarization separating surface 204a. 
P-polarized light beams having the same directions of polarization which have emerged from the polarization converting element 204 illuminate the image display element 207 provided near the focus position (the surface to be irradiated) of a condensing lens 206, through the condensing lens 206. An image displayed by the image display element 207 is projected onto a predetermined surface by a projection lens.
On the other hand, the liquid crystal projector shown in FIG. 1 forms a plurality of secondary light source images by a the fly-eye type optical integrator and superposes the light beams from the plurality of secondary light source images one upon another on the surface to be irradiated to uniformly illuminate the surface 207 to be irradiated.
The liquid crystal projector of FIG. 1, however, requires first and second lens arrays of the same degree of size as an opening in the reflecting mirror 203 and therefore, the entire apparatus tends to be bulky.
There have heretofore been proposed various liquid crystal projectors for illuminating a liquid crystal panel by a light beam from a light source, and enlarging and projecting image light such as transmitted light or reflected light from the liquid crystal panel onto a screen or a wall by a projection lens.
Usually the liquid crystal panel utilizes the polarizing characteristic of liquid crystal. Therefore, usually, polarizing filters such as a polarizer and an analyzer are provided before and behind the liquid crystal panel. The polarizing filter has the characteristic of transmitting therethrough polarized light polarized in a particular direction of polarization of incident light, and intercepting polarized light of which the direction of polarization is orthogonal thereto. Therefore, the light from the light source utilized in the liquid crystal projector has had at least a half thereof intercepted by the polarizer which is a polarizing filter and thus, the brightness of the image projected onto the screen or the wall has not been sufficient.
Projection systems being manufactured incorporating LCos SLMs currently employ PCLPs in their illumination system. The most common form of PCLP includes a reflector in the form of a light pipe having two PBSs at the input end of the light pipe. Light from the light source is incident upon one PBS. The PBS spits the light into S and P polarization states. P-polarized light is transmitted and the S-polarized light is reflected. The P-polarized light is then incident upon a rotator in the form of a half-wave retarder which is oriented such that the emerging light has its polarization axis rotated by 90 degrees such that it has been converted to S-polarized light. The S-polarized light split off by the PBS is then incident upon the second PBS where it is reflected and re-directed into the light pipe. S-polarized light thus emerges from both PBSs and is then homogenized by the light pipe. The PCLP tends to be a lower cost system than the aforementioned lenslet array PCA, but suffers from lower contrast, approximately 6:1 emerging from the PCLP as compared with the 20 to 30:1 from the lenslet array PCA, and lower efficiency; about 72% compared with approximately 80%.
Prior art U.S. Pat. No. 6,139,157 incorporated herein by reference is directed to an illuminating apparatus which appears to have several elements in common with the instant invention. A light source, light pipe, lenses, a reflective polarization beam splitter, a polarization conversion element and a target plane are all shown in this patent. Notwithstanding, the invention shown in FIG. 13 of this patent, shown herein as FIG. 2, is absent an obstruction or opaque region at an input end of the light pipe. The applicant believes that by not providing a light pipe with masked region at the input end, a uniform polarized beam will not result. By providing the obstruction or masked region at an input end of the light pipe, and ensuring that the polarization translation and separation occurs at predetermined locations in dependence upon the configuration of the masked region, a desired uniform polarized beam will result. The description of FIG. 13 is as follows:
“The polarization converting element 74 shown in FIG. 13 uses a transparent plate provided with a polarization separating surface 701 on the surface thereof as reflecting means for bending the optical path by 90.degree. and a reflecting mirror 702 on the back thereof, and the optical paths of S-polarized light reflected by the surface and P-polarized light reflected by the back are deviated in parallelism to each other, and a half wavelength plate 703 is disposed on the optical path of the S-polarized light reflected by the surface (or the optical path of the P-polarized light reflected by the back) to thereby uniformize the directions of polarization of the two lights.”
Also, in the system of prior art FIG. 13, half wavelength plates 703 are periodically provided on the flat surface of the plano-convex integrated lens 5 to thereby convert the direction of polarization of the incident S-polarized light into the same direction as the P-polarized light.”
It is unclear whether the structure shown in FIG. 13 functions as it is intended to. Since there is no structure taught which blocks light from the input end, it would appear as if all light of one linear polarization incident upon surface 701 is reflected upon the entire receiving surface of 5. It would also appear as if all light reflected from surface 702 is reflected upon the same region of 5. Therefore it is not clear how this embodiment works to selectively rotate one linear polarization and not the other.
It is an object of this invention to provide a system and method for uniformly illuminating a spatial light modulator which incorporates a novel, relatively low cost method of polarization recovery.
It is an object of this invention to provide a simple, low cost system for providing substantially uniform polarized light for use in a projection display system.